1. Field of the Invention
This invention relates to an organic thin film display element.
2. Description of the Related Art
Recently, in accordance with the development of techniques of forming organic-molecule very thin films represented by the Langmuir-Blodgett's technique ("LB technique" hereinafter), techniques for applying organic thin films to various elements have been remarkably developed. For example, the studies of MIS (metal-insulator semiconductor) elements using organic thin films, made by G. G. Roberts et al. in the Duhram University, are famous. However, elements having novel functions and effectively utilizing the properties of organic thin films have not been realized.
From the viewpoint of the application of organic thin films to elements, much attention has been paid to a charge-transfer phenomenon occurring between organic molecules. Organic materials include donor molecules having a low ionization potential and a tendency to give an electron to other molecules to become positive ions, and acceptor molecules having a high electron affinity and a tendency to receive an electron from other molecules to become negative ions. It is well known that a compound called a charge-transfer complex is formed between these two types of molecules. For instance, a compound of perylene and tetracyanoquinodimethane (TCNQ) consists of neutra molecules. On the other hand, a compound of tetramethylphenylenediamine (TMPD) and TCNQ is an ionic compound consisting of positive and negative ions. It is also known that a neutral-ionic transition phenomenon due to a change in temperature or pressure is observed in a compound of tetrathiafulvalene (TTF) and chloranil (CA) (J. B. Torrance et al Phys. Rev. Lett., 46,253 (1981).
When the charge-transfer phenomenon in organic molecules is applied to as the operational principle of a display element, it is necessary to realize that the charge-transfer is caused by an electric field or light with high efficiency and high controllability. Data of interest, relating to electrical characteristics of a charge-transfer complex, has recently been reported (Y. Tokura et al.; Manuscripts for Meeting of the Physical Society, 3a-S4-1, 3a-S4-2, 3a-S4-3, etc. (Autumn 1988); Y. Tokura et al.; Physica 143B, 527 (1986)). Namely, it is reported that in a mixed stacked compound crystal in which donor molecules and acceptor molecules are stacked with their molecular planes facing each other, the anisotropy of relative dielectric constant is high, the relative dielectric constant in the direction of stacking is very high, i.e. 100 to 1000, and non-linear electric conductivity or switching characteristics are observed under an electric field on the order of 10.sup.3 to 10.sup.4 V/cm. The reason for this is considered to be that an ionic domain formed in a neutral crystal or a neutral domain formed in an ionic crystal, thermally or electrically, is dynamically moved by an electric field.
This phenomenon, though relating to neutral-ionic transition, occurs in a very local area, and no macroscopic change appears in the whole crystal. Macroscopic neutral-ionic transition due to an electric field or light has not been realized as yet. However, in practical display elements, it is necessary that the whole crystal be changed macroscopically in accordance with the neutral-ionic transition.
In order to cause the macroscopic neutral-ionic transition in a charge-transfer complex under an electric field, it is essential to apply to the complex an electric field strength higher than the aforementioned value. Tokura et al. evaluated the aforementioned characteristics with use of a bulk crystal having a size of several millimeters in each side. In the bulk crystal, however, an electric current through the complex increases greatly in accordance with the increase in electric field strength. Thus, an electric field strength applied to the complex is limited. In order to strengthen an electric field applied to the charge-transfer complex, it is necessary to employ an element structure in which an electric current does not increase even if a high electric field is applied.